In article ,
Alden Hackmann wrote:
jim rozen writes:
First off, what surface(s) are you hand-scraping, and to what
are they fitted?
The outer rim is what gets scraped, not in the usual machine-tool scraping manner (I think, not ever having done that) but
with a heavy wood plane blade or a cabinet scraper. The reason it gets scraped is to smooth it for the contact with the
strings. The strings rest on the wheel, which is rosined and acts like a bow. To keep the tone even, the wheel must have
very little runout: if there's variation in the diameter, the string pressure on the wheel varies, and the sound changes.
Instead of getting a nice smooth "ummmmmmmmmmmmmmmmmmmmmmmmmmmmm", it sounds like "MMMMMM-mmmmmm-MMMMMMM-mmmmmm".
I've heard that -- with a buzz added from wood end-grain
extending with humidity. :-)
[ ... ]
The shaft you are using is pretty thin and flexible, that may
be where some of your chatter is coming from.
Initially when making the wheel I put the sawn blank on a faceplate to
turn it down to the basic diameter, and to drill and tap it (5/16"-18).
There is less chatter in this situation, but of course the shafts have a
little runout, and the tapped hole is never exactly perfectly square to
the blank - so between these two, there's always some runout in the
wheel when it's on the shaft.
*Tapped* hole? You mean that the wheel threads onto the shaft?
How do you tap it? In the lathe, just after drilling, while still on
the faceplate? If you go to a drill press, you're introducing an
additional place where error can come in.
I would instead of tapping it, drill it slightly undersized, and
ream to size in the lathe (or bore before reaming, to make sure that
things are on-center), and make that size a light press fit on the
shaft. Then I would use a tiny end mill, or a reciprocating saw, to
make a short radial slot from the center hole, and drill the shaft for a
pin which would be pressed in and prevent the wheel from ever rotating on
the shaft. I would have a thread on the shaft just *after* the wheel,
and a screw-on nut (perhaps with pin spanner holes) to secure the wheel
to the shaft. The shaft *should* have a shoulder to assure that the
wheel is square to the shaft, and to resist the thrust of the nut.
Consequently I make up matched sets of
wheels and shafts: once a set has been assigned to an instrument, it's
not interchangeable with another set.
Is the wheel permanently mounted to the shaft from the time it
is machined to final installation? If it comes off for intermediate
operations, and there is no pin or other feature to assure alignment
between the shaft and the wheel, each removal and replacement can
introduce more error producing runout.
LE phenolic can be somewhat abrasive so if you are using HSS
tooling it will get beaten up pretty fast. Carbide if used
instead should be *un*coated as the coatings require a honed
edge and the phenolic really wants a sharp tool and fairly
large postive rake. My personal favorite for this is polycrystaline
diamond inserts from Valenite. I run the tpg221 sized ones
for composites and they last forever and give real sharp
edge.
My past experience with carbide, at least with metal, is that the poor
old 9" SB lathe wasn't rigid enough for it. Suggestions (besides a
bigger lathe)? I'm using an Aloris QC toolpost.
1) Tighten the gibs on the compound and the cross-slide to minimize
the give there.
2) Use the largest tool shank you can for the size of toolpost
and holders. With a 9" SB, you must be using an AXA Aloris, so
the maximum shank is probably 1/2". (BXA lets you use 5/8",
buying more rigidity, but I think that is too big for a 9" SB.
3) Minimize the extension of the tool from the holder. (There is
a fancy Aloris tool holder which offers support to allow you to
clear a live center while turning. The top view is somewhat
like this:
+------+
| \
| O \
| \
| O |
| |
| O |\|
| |
| O ( 0 )
| |
| O |/|
| |
| |
+----------+
This gives you a bit more rigidity if you can't crank the
toolpost any closer to the workpiece. I use one of these when
threading using carbide insert tools, and may get another
someday.
There is also a tool holder which has clamps for insert tooling
directly on the holder -- no extra shanks to deal with. More
rigid, but it limits your choice of inserts to whatever is
supported by that holder. There are two variants, one with
positive rake, and one with negative. I have the negative rake
one since it works with the inserts which I use elsewhere. (With
chipbreaker grooves which turn it into an actual positive rake,
but you get thick tool inserts with no undercut for relief, as
that is provided by the negative rake.) Unfortunately, these
are the triangular inserts, and I don't know whether the
suggested PCD (PolyCrystaline Diamond) inserts are available in
that format. They would be a good choice, in any case. These
holders are double-ended, offering one insert for turning, and
one for facing in the same tool, only requiring transferring to
the other dovetail on the toolpost.
4) Minimize overhang in the compound. Ideally, the toolpost should
be centered over the pivot/mount of the compound to the
cross-slide.
5) Don't use the inexpensive sets of five insert holders (parallel
left, angled left, neutral, angled right and parallel right), as
they tend to be rather wimpy, and to use thinner inserts,
without a proper Carbide anvil supporting them, so they break
much more easily, and otherwise lead to chatter.
You will also find that much of the aroma from turning phenolic
goes away when cutting with a very very sharp tool. I think
most of it has to do with burning at the cutting edge.
That would be nice. ;-)
A beefier mandrel as suggested would be a good way to go.
This is how we used to make sheaves, three operations - profile
first side, profile second side, and mount on mandrel and
turn OD.
The sides are less important - just the OD (which is not square, but is
cut at several degrees angle.)
Isn't it crowned? That was how the ones which I made were
produced. (Back before I had such nice tooling and machines to work
with. :-)
Best of luck,
DoN.
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